Optical communication systems are widely used for data communication. An optical communication system may comprise an optical transmitter, a channel (e.g., optical fibers), and an optical receiver. The optical transmitter may encode data onto an optical signal, the channel may carry the optical signal from the transmitter to the receiver, and the receiver may recover the data from the received optical signal. One type of optical communication system may be a long-haul optical communication system, in which data may be carried through optical transmission links and/or optical fibers that span long distances. Long-haul optical communication systems may increase data rates by employing coherent optical transmissions, in which data may be modulated onto amplitudes, phases, and/or polarization components of an optical carrier signal.
In long-haul optical communication systems, the system performance may be limited by inter-symbol interference (ISI), which may be caused by fiber-optic impairments, such as chromatic dispersion (CD), polarization mode dispersion (PMD), phase noise, non-linear effects, etc. For example, CD may cause different spectral components (e.g., wavelengths) in a light signal to travel through an optical fiber at different speeds and arrive at a receiver at different time instants, and thus may broaden the light pulses that carry the data and lead to ISI. Some systems may compensate CD in a fiber by employing another fiber of opposite-sign dispersion, but may be at the expense of an increased non-linearity effect. Recent advances in high-speed analog-to-digital converters (ADCs) and/or digital-to-analog converters (DACs) and high performance digital signal processors (DSPs) have enabled fiber-optic impairments to be compensated digitally in DSPs.